In the preparation of papermaking pulps from lignocellulosic materials it is often desirable to bleach the pulps to obain a whitened product. Numerous methods of bleaching pulp are practiced and have been described in a variety of publications (Rapson, W. H., Editor, The Bleaching of Pulp, TAPPI Monograph Series No. 27, TAPPI, New York 1963; Singh, R. P., Editor, The Bleaching of Pulp, Third Edition, TAPPI, Atlanta 1979). Certain of these prior art bleaching processes require multiple treatment steps to remove lignin and other "color bodies" from the pulp. It is a characteristic of most bleaching processes that expensive chemicals and process equipment are required to obtain relatively small changes in the purity and brightness of the product pulp. For example, to obtain wood pulp of 90 brightness from softwood kraft pulp it is often necessary to use five stages of bleaching under conditions such as in Table I. The bleach sequence and conditions outlined in Table I, while typical, only represents one of a large number of bleach sequences in common use by the pulp industry.
TABLE I ______________________________________ Typical Reaction Conditions for SW Kraft Bleaching Temper- Active % Chemical Consistency ature Stage Chemical on Pulp % Deg. C. ______________________________________ 1 chlorine 6 3.5 35 2 alkaline 3 12 60 extraction 3 chlorine 1 12 70 dioxide 4 alkaline .5 12 60 extraction 5 chlorine .2 12 70 dioxide ______________________________________
The waste products of the bleaching process are known to contain BOD, organically bound chlorine and color. Thus, they contribute to the water pollution discharged from the pulp mill.
The efficiency of the bleaching reactions is hampered by the existence of condensation reactions. This can be particularly true in the alkaline extraction step where condensation reactions block further delignification. A publication by Seymour (Seymour, G. W., "Cost Reducing Bleach Plant Control Strategy," Seminar Notes, 1977 Bleaching Seminar on Chlorination and Caustic Extraction, TAPPI, Washington, D.C., Nov. 10, 1977) reports that the amount of caustic applied in the extraction stage can be doubled beyond normal with practically no reduction in bleach chemical usage in the following stages.
It is a continuing objective of the pulp industry to reduce overall bleaching costs by improving efficiency in the various process steps. Improved efficiency can result in lower costs by reduction of chemical usage or reduction of the number of process steps. An additional benefit of improved efficiency can be a lowering of pollutant discharge.